Optical Properties


Each pair of binoculars is defined by two numbers:

1. The first one indicates the magnification (times larger compared to the naked eye). The higher the magnification, the greater stability of the instrument is important, as the slightest vibration (or shake) will be more visible. Above a magnification of x12, it is advisable to use a tripod because the image shake is excessive.

2. The second is the diameter of the lens (or aperture) in millimeters. Plus a lens has a large diameter, the more light collection, and the observed image will have a good resolution. But beware: a large diameter implies a heavy instrument, thus more difficult to hold out long. Larger models offer exceptional views of the night sky, but must be stabilized with a tripod.

> these two separate figures of the sign x is inscribed on the body of the instrument and indicates the power. For example: 8 x 50 binoculars magnifying 8 times and with objectives of 50 mm.


it compares two different power binoculars. it is simply equal to the magnification multiplied by the diameter of the lens.

visibility factor = magnification x aperture


For example, binoculars 8 x 50 (visibility factor = 400), are more potent than 7 x 50 (FV = 350).


Vision Fields ] This is the maximum cone of vision that can embrace the eye when it is fixed. It is expressed in degrees (°), minutes of arc (‘) and second of arc (”). 1 is equivalent to 17.5 m wide at 1000 m distance.

Apparent Fields ] It depends only on the eye, their aperture and focal length. this is the field that could see through the eye for a x1 magnification. if the twins have an apparent field of view greater than 60 °, it is said that they are “wide-field”.

Field Real ] It depends this time of apparent field eyepieces and magnification of the instrument. The more it is strong, and the actual visual field is reduced. Therefore, the actual visual fields of binoculars at different magnifications are not comparable. It is sometimes register with the power of the twin (eg 15 × 45 3.7). It is obtained by dividing the apparent field of eye by the magnification of the instrument:

True field = Apparent field eyepieces / Magnification


the actual field is sometimes indicated thousand meters, for example: 56m / 1000m. this means that a thousand meters away, there is little to observe a wide real field 56 meters.

Real field 1000m = True field (°) x 17.5



It is the distance between the eye and the first eyepiece lens. A large eye relief allows a more comfortable viewing, especially for eyeglass wearers.


The exit pupil is the diameter of the light beam coming out of the binocular, to the place where it is observable. It is expressed in millimeters. It is obtained by dividing the diameter by the magnification objectives :

Exit pupil diameter (mm) = Objective diameter (mm) / Magnification (mm)


Binoculars for astronomy must have an exit pupil between 4 and 7 mm. The more it is, the more the instrument is bright. For example, for binoculars of 10 x 50, the exit pupil is 5mm. But this corresponds to the diameter of the pupil dilated to its maximum by an adult in night vision. This pair of binoculars will be well suited for astronomy, because the eye receives all the light collected by the instrument. The eye of a child or young teenager can open it up to 7 mm. in this case, binoculars or 8 x 56 7 x 50 will be better suited. In addition, a large exit pupil facilitates the observation of an object when the observer is in an unstable position (eg on a ship).


we obtain the luminosity index by squaring the ratio between the diameter and magnification objectives, or by squaring the exit pupil.

Luminosity = (objective diameter / Magnification)² = (Exit pupil)²


The more the brightness of the instrument is large, and the brighter is the image.


The twilight factor is calculated by taking the square root of the magnification and diameter. For 8×42 binoculars, it is therefore of 8×42 root, that is to say the root 336 which is equal to 18.3.

Twilight factor = √(Magnification x Objective diameter)


The more twilight factor will be high, and the more the image will have a good resolution.

> Meaning of letters written after the power binoculars

WP> means “water-tight” (water proof). The waterproof binoculars are made with special seals and vacuum inside the instrument is filled with nitrogen so that water can not penetrate.
B> (on german models) means “suitable for eyeglass wearers.” These binoculars therefore have a large eye relief.
G, GA or RA > means “rubber-wrapped” (armed rubber). The rubber coating protects the binoculars against shocks and water splashes.
CF> sometimes means “Central Focus”, but sometimes “Close Focus”.
(“central focus” means the center focus. “CloseFocus” means the minimum focusing distance is very short, usually between 2 and 4 m, whereas with most binoculars, development is only possible from 5 meters.)
W, WF, WW or Wide> “wide angle” or “wide field”.
UC> “Ultra Compact”. This therefore means particularly compact and lightweight binoculars (usually aluminum and sometimes titanium).
HP> “High eye Point”. These binoculars offer a wide opening at the exit pupil.
D> (on german models) means “roof prism”.
IS> “Image Itabilisator”. These binoculars feature an image stabilizer.

Glasses Transparency


because glass surfaces reflect some of the light that passes through a pair of binoculars designed without anti-reflective coating on the lenses and its prisms produce a dark image and poor quality. optical binoculars may be covered with one or more anti-reflective layers which improve their transparency. in general, the more processed lens and prism surfaces, the more the image is sharp and clear, and the price of binoculars will be.

There are different levels of anti-glare treatment :

► Treated : A single surface air-glass is coated with a single layer of anti-reflective material .
► All surfaces treated (or fully processed) : All air-glass surfaces are coated with a single anti-reflection layer.
► Multilayer treated : At least one air-glass surface is coated with a multilayer coating, or other surfaces are coated with a single layer anti-reflection or untreated.
► Treated multi-coated : all air-glass surfaces are coated with a multilayer coating.
► Phase coating: some top quality binoculars are specified “with phase treatment.” this is a special treatment layer applied on the roof prism.

When a light beam hits the roof prism, it is reflected several times in the form of a roof ridge. In addition, the reflection divides the beam into two segments, which results in a shift of the phases of the light waves. this has the effect of reducing both the resolution contrast. to avoid this loss, we apply specific processing layers, called treatment phase, which correct the phenomenon described.


There are two types of glass:

► BK7(entry level) 60% of light transmitted
► barium BAK4 (high-range) 95% of light transmitted

BK7 (borosilicate) is the standard quality binoculars glasses. these are cheaper to manufacture and therefore more used for entry binoculars. The glass BAK4 (barium sulfate) are extremely clear and transparent, and are considered to have the best quality on the market. The BAK4 quality is the result of a very complicated and long manufacturing process.